The RAWDNAT target will rewrite the destination address in the IP header, much like the NETMAP target.

--to-destination addr[/mask]
    Network address to map to. The resulting address will be constructed the following way: All "one" bits in the mask are filled in from the new address. All bits that are zero in the mask are filled in from the original address.

See the RAWSNAT help entry for examples and constraints.